The emission of pollutants such as sulfur oxides and nitrogen oxides in combustion waste gases causes serious environmental problems. Major efforts are underway to reduce these emissions through the implementation of particulate filters and NOx conversion devices. One promising approach to minimizing NOx emissions involves capturing and storing NOx as an alkali or alkaline earth nitrate during normal operation (lean conditions) and releasing the stored NOx after reducing it to molecular nitrogen (N2) under fuel-rich conditions.
Sulfur oxides (SOx) are produced as combustion byproducts and interfere with the function of current NOx traps by reacting with NOx catalytic components, degrading or “poisoning” the trap. Sulfur dioxide also reacts with the oxidants present in particulate filters, yielding sulfur trioxide, sulfuric acid particulates and depositing sulfate salts on the catalyst, which degrades the effectiveness of the particulate filter. Despite the recent introduction of low sulfur diesel fuels, the 15 ppmw (parts per million by weight) concentration of sulfur in these fuels still overwhelms current NOx traps and particulate filters.
Current sulfur absorbent technology has significant limitations. For example, copper-doped alumina (Cu—Al2O3) has been studied as a regenerable flue gas sulfur oxide absorbent. It reacts with SO2 and O2 at ˜350° C. to form CuSO4 and Al2(SO4)3. The sulfated absorbent can be regenerated by reduction in H2 or CH4 at 400-500° C., followed by oxidation in air at 500° C. to reform the copper oxide phase. The chemistry involved in the absorption/regeneration cycle makes it extremely challenging to use Cu-doped Al2O3 as an on-line regenerable sulfur trap for diesel emission aftertreatment systems, due to the copper oxide/copper metal redox that is taking place in parallel with the sulfate adsorption and desorption. The unsulfated CuO will react with rich gas to form metallic Cu, which not only causes a fuel penalty, but also prevents the system from fast regeneration because copper oxide reduction competes kinetically with copper sulfate reduction. Thus, high loadings of copper do not provide an advantage in the preparation of the absorbent.